An alloy commonly used for large pressure vessels, known as SA508 Grade 3, has a microstructure after heat treatment consisting of a mixture of tempered bainite and martensite at fast cooled regions near surfaces subject to water quenching. These two phases are conventionally recognized to consist of fine platelets, each of which is approximately 0.2 μm in thickness, enhancing strength and leading to good toughness properties. We have discovered in our experimental work that there are circumstances where the adjacent platelets of a similar orientation can coalesce as the austenite transforms, to produce much coarser structures which are believed to be detrimental to toughness. An examination of published micrographs reveals that such coalesced regions existed but were not noticed in previous studies. The mechanism of coalescence is described and methods to ameliorate the coarsening are discussed.

References

References
1.
Cogswell
,
D.
,
Swan
,
D.
,
Mitchell
,
R.
, and
Garwood
,
S.
,
2010
, “
Materials and Structural Integrity Challenges for the Next Nuclear Generation
,”
63rd Annual Assembly and International Conference of the International Institute of Welding
.
2.
Lee
,
B.
,
Hong
,
J.
,
Yang
,
W.
,
Huh
,
M.
, and
Chi
,
S.
,
2000
, “
Master Curve Characterization of the Fracture Toughness in Unirradiated and Irradiated RPV Steels Using Full- and 1/3-Size Pre-Cracked Charpy Specimens
,”
Int. J. Pressure Vessels Piping
,
77
, pp.
599
604
.10.1016/S0308-0161(00)00032-6
3.
Bhadeshia
,
H. K. D. H.
,
Keehan
,
E.
,
Karlsson
,
L.
, and
Andrén
,
H. O.
,
2006
, “
Coalesced Bainite
,”
Trans. Indian Inst. Metals
,
29
, pp.
689
694
. Available at: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.160.6583&rep=rep1&type=pdf
4.
Pak
,
J. H.
,
Bhadeshia
,
H. K. D. H.
, and
Karlsson
,
L.
,
2012
, “
Mechanism of Misorientation Development Within Coalesced Martensite
,”
Mater. Sci. Technol.
,
28
(
8
), pp.
918
923
.10.1179/1743284712Y.0000000023
5.
Pak
,
J. H.
,
Bhadeshia
,
H. K. D. H.
,
Karlsson
,
L.
, and
Keehan
,
E.
,
2008
, “
Coalesced Bainite by Isothermal Transformation of Reheated Weld Metal
,”
Sci. Technol. Weld. Joining
,
13
(
7
), pp.
593
597
.10.1179/136217108X338926
6.
Caballero
,
F. G.
,
Chao
,
J.
,
Cornide
,
J.
,
García-Mateo
,
C.
,
Santofimia
,
M. J.
, and
Capdevila
,
C.
,
2009
, “
Toughness Deterioration in Advanced High Strength Bainitic Steels
,”
Mater. Sci. Eng. A
,
525
, pp.
87
95
.10.1016/j.msea.2009.06.034
7.
Keehan
,
E.
,
Bhadeshia
,
H. K. D. H.
, and
Thuvander
,
M.
,
2008
, “
Electron Backscatter Diffraction Study of the Coalesced Bainite i High Strength Steel Weld Metals
,”
Mater. Sci. Technol.
,
24
(
10
), pp.
1183
1188
.10.1179/174328407X226572
8.
Keehan
,
E.
,
Karlsson
,
L.
, and
Andrén
,
H. O.
,
2006
, “
Influence of Carbon, Manganese and Nickel on Microstructure and Properties of Strong Steel Weld Metal. Part 1—Effect of Nickel Content
,”
Sci. Technol. Weld. Joining
,
11
(
1
), pp.
1
8
.10.1179/174329306X77830
9.
Suzuki
,
K.
,
1982
, “
Neutron Irradiation Embrittlement of ASME SA508 Cl. 1 Steel
,”
J. Nucl. Mater.
,
109
, pp.
443
450
.10.1016/0022-3115(82)90515-3
10.
Park
,
S. G.
,
Kim
,
M. C.
,
Lee
,
B. S.
, and
Wee
,
D. M.
,
2010
, “
Correlation of the Thermodynamic Calculation and the Experimental Observation of Ni-Mo-Cr Low Alloy Steel Changing Ni, Mo, and Cr contents
,”
J. Nucl. Mater.
,
407
, pp.
126
135
.10.1016/j.jnucmat.2010.09.004
11.
Hawthorne
,
J.
,
1985
, “
Composition Influences and Interactions in Radiation Sensitivity of Reactor Vessel Steels
,”
Nucl. Eng. Des.
,
89
, pp.
223
232
.10.1016/0029-5493(85)90156-6
12.
Chang
,
L. C.
, and
Bhadeshia
,
H. K. D. H.
,
1996
, “
Microstructure of Lower Bainite Formed at Large Undercooling Below Bainite Start Temperature
,”
Mater. Sci. Technol.
,
12
, pp.
233
236
.10.1179/026708396790165687
13.
Pak
,
J. H.
,
2012
, “
Coalesced Bainite and Martensite
,” Ph.D. thesis, GIFT, POSTECH, Pohang, South Korea.
14.
Christian
,
J. W.
,
1958
, “
Accommodation Strains in Martensite Formation, the Use of the Dilatation Parameter
,”
Acta Metall.
,
6
, pp.
377
379
.10.1016/0001-6160(58)90077-4
15.
Bhadeshia
,
H. K. D. H.
,
David
,
S. A.
,
Vitek
,
J. M.
, and
Reed
,
R. W.
,
1991
, “
Stress Induced Transformation to Bainite in Fe-Cr-Mo-C Pressure Vessel Steel
,”
Mater. Sci. Technol.
,
7
, pp.
686
698
.10.1179/026708391790184915
16.
Pak
,
J.
,
Suh
,
D. W.
, and
Bhadeshia
,
H. K. D. H.
,
2012
, “
Promoting the Coalescence of Bainite Platelets
,”
Scr. Mater.
,
66
, pp.
951
953
.10.1016/j.scriptamat.2012.02.041
17.
Pous-Romero
,
H.
,
Lonardelli
,
I.
,
Cogswell
,
D.
, and
Bhadeshia
,
H. K. D. H.
,
2013
, “
Austenite Grain Growth in a Nuclear Pressure Vessel Steel
,”
Mater. Sci. Eng., A
,
567
, pp.
72
79
.10.1016/j.msea.2013.01.005
18.
Yang
,
H. S.
, and
Bhadeshia
,
H. K. D. H.
,
2007
, “
Uncertainties in Dilatometric Determination of Martensite Start Temperature
,”
Mater. Sci. Technol.
,
23
(
5
), pp.
556
560
.10.1179/174328407X176857
19.
Peet
,
M.
, and
Bhadeshia
,
H. K. D. H.
,
1982
, “
Software for Transformations in Steels
.” Available at: http://www.msm.cam.ac.uk/map/steel/programs/mucg83.html
20.
Kim
,
S.
,
Kang
,
S. Y.
,
Oh
,
S. J.
,
Kwon
,
S.
,
Lee
,
S.
,
Kim
,
J. H.
, and
Hong
,
J. H.
,
2000
, “
Correlation of the Microstructure and Fracture Toughness of the Heat-Affected Zones of an SA508 Steel
,”
Metall. Mater. Trans. A
,
31A
, pp.
1107
1119
.10.1007/s11661-000-0106-2
21.
Kim
,
S.
,
Im
,
Y.-R.
,
Lee
,
S.
,
Lee
,
H.-C.
,
Kim
,
S.-J.
, and
Hong
,
J. H.
,
2004
, “
Effects of Alloying Elements on Fracture Toughness in the Transition Temperature Region of Base Metals and Simulated Heat-Affected Zones of Mn-Mo-Ni Low Alloy Steels
,”
Metall. Mater. Trans.
,
35A
, pp.
2027
2037
.10.1007/s11661-004-0151-3
22.
Kim
,
S.
,
Im
,
Y.-R.
,
Lee
,
S.
,
Lee
,
H.-C.
,
Oh
,
Y. J.
, and
Hong
,
J. H.
,
2001
, “
Effects of Alloying Elements on Mechanical and Fracture Properties of Base Metals and Simulated HAZ of SA508 Steels
,”
Metall. Mater. Trans. A
,
32A
, pp.
903
911
.10.1007/s11661-001-0347-8
23.
Jang
,
H.
,
Kim
,
J.-H.
,
Jang
,
C.
,
Lee
,
J. G.
, and
Kim
,
T. S.
,
2013
, “
Low-Cycle Fatigue Behaviours of Two Heats of SA508 Gr. 1a Low Alloy Steel in 310 °C Air Deoxygenated Water—Effects of Dynamic Strain Aging and Microstructures
,”
Mater. Sci. Eng., A
,
580
, pp.
41
50
.10.1016/j.msea.2013.05.049
24.
ASTM
,
2004
, “
Standard Specification for Quenched and Tempered Vacuum-treated Carbon and Alloy Steel Forgings for Pressure Vessel Components A508A/A548M
.”
You do not currently have access to this content.